I think a 4' x 8' coop would be fine (32 square feet) for the amount of ducks you want/have. Rule of thumb is between 3 - 5 square feet per duck. My coop is 24 square feet (3' x 8' x 4') for five ducks (3 SAs, 2 WH and 1 PEK).
If you're interested, here's a link to a video providing an overview of the coop and duck yard I built.
smithmal
If you're interested, here's a link to a video providing an overview of the coop and duck yard I built.
smithmal